The general features of the proteolytic fragmentation of fibrinogen have been established by our work and others previous work. Further studies will focus on details, as the fate of the thrombin susceptible bonds during the fragmentation. Some physicochemical studies will be performed to characterize the isolated D and E fragments. The data on their unfolding by heat, obtained by differential scanning calorimetry, will be supplemented by studies of unfolding by acid, alkali, and denaturing agents, using difference spectroscopy and fluorescence for this purpose. The structural rigidity of the native fibrinogen molecule will be studied by nanosecond fluorimetry of derivatives labeled by fluorescent chromophores.